The proposed research is an extension of our previous research project on the mathematical modeling of particle deposition in the human lung airways. The emphases for the current phase of the program will be placed on: 1. the use of a realistic respiratory tract geometry, 2. the development of accurate deposition formulas, 3. extensive comparison of the predicted results with experimental data, and 4. the development of a stochastic deposition model capable of predicting intersubject variability.